<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Version>2.4.0.8</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>90</ContextCount>
  <ElementCount>223</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>36</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>101 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>103 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/StatementOfFinancialPositionClassified</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>104 - Statement - Condensed Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/StatementOfFinancialPositionClassifiedParenthetical</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>105 - Statement - Condensed Consolidated Statements of Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/StatementOfOtherComprehensiveIncome</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Loss</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>106 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/StatementOfCashFlowsIndirect</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>107 - Disclosure - Nature of the Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsNatureOfOperations</Role>
      <ShortName>Nature of the Business</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>108 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsOrganizationConsolidationBasisOfPresentationBusinessDescriptionAndAccountingPoliciesTextBlock</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>109 - Disclosure - Net Loss Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsEarningsPerShareTextBlock</Role>
      <ShortName>Net Loss Per Common Share</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>110 - Disclosure - License and Collaboration Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsCollaborativeArrangementDisclosureTextBlock</Role>
      <ShortName>License and Collaboration Agreements</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>111 - Disclosure - Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsFairValueDisclosuresTextBlock</Role>
      <ShortName>Fair Value of Financial Instruments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>112 - Disclosure - Accounts Payable, Accrued Expenses and Other</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsAccountsPayableAndAccruedLiabilitiesDisclosureTextBlock</Role>
      <ShortName>Accounts Payable, Accrued Expenses and Other</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>113 - Disclosure - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsDebtDisclosureTextBlock</Role>
      <ShortName>Borrowings</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>114 - Disclosure - Common Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsStockholdersEquityNoteDisclosureTextBlock</Role>
      <ShortName>Common Stock</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>115 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsDisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock</Role>
      <ShortName>Stock-Based Compensation</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>116 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsCommitmentsAndContingenciesDisclosureTextBlock</Role>
      <ShortName>Commitments and Contingencies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>117 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsSubsequentEventsTextBlock</Role>
      <ShortName>Subsequent Events</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>118 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsOrganizationConsolidationBasisOfPresentationBusinessDescriptionAndAccountingPoliciesTextBlockPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>119 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsOrganizationConsolidationBasisOfPresentationBusinessDescriptionAndAccountingPoliciesTextBlockTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>120 - Disclosure - Net Loss Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsEarningsPerShareTextBlockTables</Role>
      <ShortName>Net Loss Per Common Share (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>121 - Disclosure - License and Collaboration Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsCollaborativeArrangementDisclosureTextBlockTables</Role>
      <ShortName>License and Collaboration Agreements (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>122 - Disclosure - Fair Value of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsFairValueDisclosuresTextBlockTables</Role>
      <ShortName>Fair Value of Financial Instruments (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>123 - Disclosure - Accounts Payable, Accrued Expenses and Other (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsAccountsPayableAndAccruedLiabilitiesDisclosureTextBlockTables</Role>
      <ShortName>Accounts Payable, Accrued Expenses and Other (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>124 - Disclosure - Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsDebtDisclosureTextBlockTables</Role>
      <ShortName>Borrowings (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>125 - Disclosure - Common Stock (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsStockholdersEquityNoteDisclosureTextBlockTables</Role>
      <ShortName>Common Stock (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>126 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/NotesToFinancialStatementsDisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlockTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>127 - Disclosure - Nature of the Business - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureNatureOfTheBusinessAdditionalInformation</Role>
      <ShortName>Nature of the Business - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>128 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformation</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>129 - Disclosure - Summary of Significant Accounting Policies - Schedule of Non-Controlling Interest in Subsidiary (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesScheduleOfNonControllingInterestInSubsidiary</Role>
      <ShortName>Summary of Significant Accounting Policies - Schedule of Non-Controlling Interest in Subsidiary (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>130 - Disclosure - Summary of Significant Accounting Policies - Schedule of Available-for-Sale Securities, All of Which Have Maturities of Twelve Months or Less (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesScheduleOfAvailableforSaleSecuritiesAllOfWhichHaveMaturitiesOfTwelveMonthsOrLess</Role>
      <ShortName>Summary of Significant Accounting Policies - Schedule of Available-for-Sale Securities, All of Which Have Maturities of Twelve Months or Less (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>131 - Disclosure - Summary of Significant Accounting Policies - Schedule of Available-for-Sale Securities in an Unrealized Loss Position (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesScheduleOfAvailableforSaleSecuritiesInAnUnrealizedLossPosition</Role>
      <ShortName>Summary of Significant Accounting Policies - Schedule of Available-for-Sale Securities in an Unrealized Loss Position (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>132 - Disclosure - Net Loss Per Common Share - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureNetLossPerCommonShareAdditionalInformation</Role>
      <ShortName>Net Loss Per Common Share - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>133 - Disclosure - Net Loss Per Common Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureNetLossPerCommonShareScheduleOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedWeightedAverageShares</Role>
      <ShortName>Net Loss Per Common Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>134 - Disclosure - License and Collaboration Agreements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureLicenseAndCollaborationAgreementsAdditionalInformation</Role>
      <ShortName>License and Collaboration Agreements - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>135 - Disclosure - License and Collaboration Agreements - Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureLicenseAndCollaborationAgreementsScheduleOfRevenueRecognizedAndAssetsAndLiabilitiesUnderCollaborativeArrangements</Role>
      <ShortName>License and Collaboration Agreements - Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>136 - Disclosure - Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureFairValueOfFinancialInstrumentsScheduleOfAssetsAndLiabilitiesMeasuredAtFairValueOnARecurringBasis</Role>
      <ShortName>Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>137 - Disclosure - Fair Value of Financial Instruments - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureFairValueOfFinancialInstrumentsAdditionalInformation</Role>
      <ShortName>Fair Value of Financial Instruments - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>138 - Disclosure - Accounts Payable, Accrued Expenses and Other - Schedule of Accounts Payable, Accrued Expenses and Other (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureAccountsPayableAccruedExpensesAndOtherScheduleOfAccountsPayableAccruedExpensesAndOther</Role>
      <ShortName>Accounts Payable, Accrued Expenses and Other - Schedule of Accounts Payable, Accrued Expenses and Other (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>139 - Disclosure - Borrowings - Schedule of Future Minimum Payments under the Loans Payable (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureBorrowingsScheduleOfFutureMinimumPaymentsUnderTheLoansPayable</Role>
      <ShortName>Borrowings - Schedule of Future Minimum Payments under the Loans Payable (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>140 - Disclosure - Borrowings - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureBorrowingsAdditionalInformation</Role>
      <ShortName>Borrowings - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>141 - Disclosure - Common Stock - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureCommonStockAdditionalInformation</Role>
      <ShortName>Common Stock - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>142 - Disclosure - Common Stock - Schedule of Shares Reserved for Future Issuance (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureCommonStockScheduleOfSharesReservedForFutureIssuance</Role>
      <ShortName>Common Stock - Schedule of Shares Reserved for Future Issuance (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>143 - Disclosure - Stock-Based Compensation - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureStockBasedCompensationAdditionalInformation</Role>
      <ShortName>Stock-Based Compensation - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>144 - Disclosure - Stock-Based Compensation - Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureStockBasedCompensationScheduleOfAssumptionsUsedToCalculateFairValueOfOptionsGrantedToEmployees</Role>
      <ShortName>Stock-Based Compensation - Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>145 - Disclosure - Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureStockBasedCompensationScheduleOfRecognizedStockBasedCompensationExpense</Role>
      <ShortName>Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>146 - Disclosure - Stock-Based Compensation - Summary of Stock Option Activity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureStockBasedCompensationSummaryOfStockOptionActivity</Role>
      <ShortName>Stock-Based Compensation - Summary of Stock Option Activity (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>147 - Disclosure - Commitments and Contingencies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformation</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>148 - Disclosure - Subsequent Events - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merrimackpharma.com/taxonomy/role/DisclosureSubsequentEventsAdditionalInformation</Role>
      <ShortName>Subsequent Events - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">Element us-gaap_ResearchAndDevelopmentExpense had a mix of decimals attribute values: -5 -3.</Log>
    <Log type="Info">Element us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross had a mix of decimals attribute values: -5 -3.</Log>
    <Log type="Info">'Monetary' elements on report '128 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '134 - Disclosure - License and Collaboration Agreements - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '140 - Disclosure - Borrowings - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Shares' elements on report '141 - Disclosure - Common Stock - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Shares' elements on report '143 - Disclosure - Stock-Based Compensation - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">Process Flow-Through: 103 - Statement - Condensed Consolidated Balance Sheets</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Mar. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 31, 2012'</Log>
    <Log type="Info">Process Flow-Through: 104 - Statement - Condensed Consolidated Balance Sheets (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 105 - Statement - Condensed Consolidated Statements of Comprehensive Loss</Log>
    <Log type="Info">Process Flow-Through: 106 - Statement - Condensed Consolidated Statements of Cash Flows</Log>
  </Logs>
  <InputFiles>
    <File>mack-20140331.xml</File>
    <File>mack-20140331.xsd</File>
    <File>mack-20140331_cal.xml</File>
    <File>mack-20140331_def.xml</File>
    <File>mack-20140331_lab.xml</File>
    <File>mack-20140331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
